Shellfish could supplant tree-ring climate data

Mussels could yield a more accurate temperature record than trees.Alamy

Oxygen isotopes in clamshells may provide the most detailed record yet of global climate change, according to a team of scientists who studied a haul of ancient Icelandic molluscs.

Most measures of palaeoclimate provide data on only average annual temperatures, says William Patterson, an isotope chemist at the University of Saskatchewan in Saskatoon, Canada, and lead author of the study1. But molluscs grow continually, and the levels of different oxygen isotopes in their shells vary with the temperature of the water in which they live. The colder the water, the higher the proportion of the heavy oxygen isotope, oxygen-18.

The study used 26 shells obtained from sediment cores taken from an Icelandic bay. Because clams typically live from two to nine years, isotope ratios in each of these shells provided a two-to-nine-year window onto the environmental conditions in which they lived.

Patterson's team used a robotic sampling device to shave thin slices from each layer of the shells' growth bands. These were then fed into a mass spectrometer, which measured the isotopes in each layer. From those, the scientists could calculate the conditions under which each layer formed.

"What we're getting to here is palaeoweather," Patterson says. "We can reconstruct temperatures on a sub-weekly resolution, using these techniques. For larger clams we could do daily."

It's an important step in palaeoclimatic studies, he says, because it allows scientists to determine not only changes in average annual temperatures, but also how these changes affected individual summers and winters.

"We often make the mistake of saying that mean annual temperature is higher or lower at some period of time," Patterson says. "But that is relatively meaningless in terms of the changes in seasonality."

Click on image for a larger version.Patterson, W. P. et al, PNAS

For example, in early Norse Iceland — part of the 2,000-year era spanned by the study — farmers were dependent on dairy farming and agriculture. "For a dairy culture, summer is by far the most important," he says. "A one-degree decrease in summer temperatures in Iceland results in a 15% decrease in agricultural yield. If that happens two years in a row, your family's wiped out."

Technically, the molluscs record water temperatures, not air temperatures. But the two are closely linked — specially close to the shore, where most people lived. "So, when the water temperatures are up, air temperatures are up. When water temperatures are down, air temperatures are down," Patterson says.

Lean times

One of Patterson's goals was to verify assertions in historical Icelandic sagas describing the weather. Because these sagas include dispatches to the king back in Norway, there was an incentive to exaggerate. "If you're having a bad year, you're not expected to provide as much in the way of tribute or taxes," Patterson explains.

The study's findings suggest that the sagas are reasonably accurate. In the 1000s, for example, the 'Book of Settlements' — a medieval manuscript containing details of Iceland's settlements — reports a famine so severe "men ate foxes and ravens" and "the old and helpless were killed and thrown over cliffs", Patterson says. And according to his shells, it was indeed a difficult era, with summer water temperatures peaking at only 5–6 °C, down from as high as 7.5–9.5 °C around 100 years earlier.

The mollusc-based temperature record for Iceland suggests Norse sagas can be trusted.Mary Evans Picture Library

Patterson's data also reveal a number of climate changes recorded by historians, including a Roman-era warming period, a cold snap in the Dark Ages and a subsequent period of warming, during which the Vikings discovered Iceland.

But it's not just historians who will be interested. The new data will help climate modellers to improve their understanding of seasonal effects in the North Atlantic, Patterson says. "This is a new line of evidence."

Other scientists are impressed. "The technique is fascinating," says geoscientist Richard Alley of Pennsylvania State University in University Park. "This represents a huge amount of work, and demonstrates outstanding potential for revealing past processes in unprecedented detail. One can envision a tree-ring-like continuous history, given a lot more effort."

If he can find the funding, that is exactly what Patterson would like to establish next. "We have what may be the world's oldest clam," he says, "that might give a continuous record going back 400 years." He also wants to push the study back towards the end of the last ice age. "We have 11,000 years worth of material," he says.